Fluid catalytic cracking (FCC) is a hydrocarbon conversion process accomplished by contacting hydrocarbons in a fluidized reaction zone with a catalyst. As the cracking reaction proceeds substantial amounts of highly carbonaceous material referred to as coke are deposited on the catalyst. A high temperature regeneration operation within a regeneration zone combusts coke from the catalyst. Coke-containing catalyst, referred to herein as coked catalyst, is continually removed from a reactor and replaced by essentially coke-free catalyst from a regenerator.
In the regenerator, the coke is burned from the catalyst with oxygen-containing gas, usually air. Flue gas formed by burning coke in the regenerator may be treated for removal of particulates and heat recovery by oxidation of carbon monoxide. The main goal of the regenerator is to burn the coke off the catalyst, so high coke burn efficiency i.e. combusting most of the coke in shorter residence time, is preferred because it will reduce the equipment size, operational cost and emission levels.
After burn is a phenomenon that occurs when hot flue gas that has been separated from regenerated catalyst contains carbon monoxide that combusts to carbon dioxide in a dilute phase of catalyst containing oxygen. Incomplete combustion of coke to carbon dioxide can result from insufficient oxygen in the combustion gas, poor fluidization or aeration of the coked catalyst in the regenerator vessel or poor distribution of coked catalyst in the regenerator vessel. The heat from after burn can be detrimental to regenerator equipment.
Because FCC units that process heavy residue feed generate more heat than is needed to vaporize feed and to promote the cracking reaction it is desirable to control the regeneration temperature and heat release to the reactor. The two most common ways to control regeneration temperature are to control the ratio of carbon dioxide to carbon monoxide and to use catalyst coolers to generate steam and cool the catalyst. It is most economical to run at the highest carbon monoxide concentration possible in the flue gas to recovery heat from the flue gas in a downstream CO boiler. However, operating at a low CO2-to-CO ratio carries the risk of after burn and uncombusted coke left on catalyst.
Several types of catalyst regenerators are in use today. A conventional bubbling bed regenerator typically has just one section in which air is bubbled through a dense catalyst bed. Coked catalyst is added, and regenerated catalyst is withdrawn from the same dense catalyst bed. In order to maximize the regenerated catalyst activity at a given make up catalyst rate, the carbon on catalyst must be reduced to a minimum.
Most modern residue fluid cracking units use a two-stage bubbling bed regenerator to finish the catalyst clean up and reduce the carbon on catalyst to a minimum. Two-stage bubbling beds have two sections. Coked catalyst is added to a dense bed in an upper, first section and is partially regenerated with air in flue gas from a second stage. The partially regenerated catalyst is transported to a dense bed in a lower, second section and completely regenerated with air. The completely regenerated catalyst is withdrawn from the second section. The second stage is generally operated in complete combustion where all carbon monoxide is converted to carbon dioxide and an excess of oxygen is present in the flue gas.
In a one or two-stage fluidized bubbling bed regenerator, catalyst lifted upwardly by air distributed into the regenerator falls non-uniformly in a phenomenon called back mixing. In bubbling beds, the catalyst phase is back mixed from top to bottom while the gas phase is nearly plug flow with a high oxygen concentration at the bottom and low oxygen concentration at the top. Back mixing causes the residence time to increase and the combustion rate to be non-uniform which can generate hot spots, increase catalyst deactivation and reduce combustion efficiency. Back mixing also lowers the catalyst bed density thereby increasing the equipment size.
FCC regenerators are large in size and costly to build. They are large because of the requirement to stage air supply to burn large amounts of coke on spent catalyst. Without staging the combustion of coke is likely to generate enough heat to destroy the zeolite framework of the catalyst and render it inactive.
Therefore, there is a need for improved processes and apparatuses for efficiently regenerating catalyst while preventing after burn and back mixing. There is a need for a process and an apparatus to better control coke and oxygen concentration and temperature profiles in a regenerator which promotes more efficient combustion of coke from catalyst. Further, there is a need for an apparatus that improves FCC regenerator efficiency and reduces vessel size.